We had a QB battle all through spring and fall camp, to the point leading up to the start of the season we still didn't have a QB picked. The coaches did a straw poll with the players asking them which QB they preferred, and the overwhelming majority picked Devin Brown. Of course Day's decision overrides everything and he stuck with McCord, presumably due to him being a year older, more experienced, and a slightly more understanding of the playbook. Devin Brown was the younger, more mobile, higher ceiling player that might turn the ball over more or make those young guy mistakes.
There's also rumors of McCord's dad being one of those kinds of dads, that think their son deserves all the reps, all the snaps, etc. It's been said by OSU insiders that McCord entered the portal because Day could not promise McCord's dad that his son would be the starter in the spring.